The goal of this project is to elucidate mechanistic details of the reaction pathway of acireductone dioxygenase (ARD), a nickel enzyme found in the methionine salvage pathway. Key unanswered questions regarding the role of the nickel center in catalysis include: (1) How does the acireductone substrate bind to the Ni(ll) center in the enzyme/substrate (ES) complex? (2) Is unpaired spin density from the Ni(ll) delocalized on to the bound acireductone substrate? (3) To what degree does the Lewis acidity of the Ni(ll) center influence the ARD reaction? (4) How do the primary and secondary coordination environments of the Ni(ll) ion influence spin delocalization and 02 reactivity of the ES complex? The proposed experiments are designed to test hypotheses regarding structure/function relationships in the ES complex of ARD. The first specific aim focuses on the generation, identification, isolation, and structural characterization of mononuclear Ni(ll) complexes that have a bound acireductone (or similar) substrate and are chelated by N4- donor ligands of varying secondary structure. A wide array of physical methods will used to characterize new complexes to allow comparison to enzymatic species. A key method of characterization will be paramagnetic 1H NMR, which will provide information regarding unpaired spin delocalization in a Ni(ll) coordinated acireductone substrate. Reactivity studies will follow and will involve treatment of isolated or in-situ generated Ni(ll)-acireductone complexes with 02. These reactions will be probed using the tools of mechanistic inorganic chemistry, including spectroscopic studies, product analysis, isotope labeling, and kinetic measurements. In a second specific aim, the influence of a Ni(ll)-O(carboxylate) interaction on the stability and reactivity of the ARD ES complex will be probed via investigation of the formation and 02 reactivity of synthetic Ni(ll) model ES complexes supported by chelate N30-donor ligands. This work is important to human health because human cancer tumor cells can exhibit "methionine dependence" and the methionine salvage pathway may be involved in regulation of methionine levels in vivo.